1.1 (B) Shows The Transformational Stereoisomer Of Butane
Conformational stereoisomer of butane
Butane, C4H10 is an alkane with four carbon atoms which are bonded to each other by single bonds. Butane can form two isomers which are n-butane and isobutane. Figure1.1(a) and 1.1(b) shows the structures of n-butane and isobutane.
Alkanes usually show conformational isomerism due the presence of C-C single bonds. It is important to understand that conformational isomer can only occur in compound with single bond but not double or triple bond. This is because only single bond allows free rotation among the atoms as it has lower energy. Compare to double bond which has higher energy that cause restriction of rotation, breaking the bond to allow rotation is mostly impossible.
The rotations of isobutane on the axis of C-C bond can be seen in many angles’ …show more content…
degrees, there are 60°, 120°, 180°, 240°, 300° and 0°, 360° and each angle of rotation give rise to different conformational isomers. For example, eclipsed, gauche, and anti-butane are all conformational isomer to each other. See Table 1.2, it shows different conformational isomers of butane with their angle’s degree.
The conformational isomers of butane can interconvert to each other around the central carbon single bond. For example, the fully-eclipsed conformation butane rotates 180° to form anti-conformation butane and further rotate 90° to form gauche conformation butane. See figure 1.3. The rotation sequence is based on 60° rotation on the C-C bond where fully eclipsed conformation is convert to gauche conformation, then convert to eclipsed conformation, followed by anti-conformation, then back to eclipsed conformation, gauche conformation, fully eclipsed conformation and so on.
The stability of the conformational isomers of butane is depends on the degrees of rotation, where the further the identical molecule groups, the more stable the conformation due to reduce of steric hindrance between the molecules.
The stability of the conformations can be explained by focusing on staggered and eclipsed conformers. Staggered conformers are with dihedral 60 degrees, and the hydrogen atoms of C-1 are far from the hydrogen atoms of C-2. Staggered conformers include anti-conformation and gauche conformation. Eclipsed conformers are which the hydrogen atoms of C-1 are lined up with the hydrogen atoms of C-2. Refer back to Table 1.2 which clearly shows the structure explain the staggered and eclipsed conformers. Staggered conformers are more stable than eclipsed conformers due to distribution of hydrogen atoms which can lead to steric hindrance. The hydrogen atoms in staggered conformers are further away from one another compare to the hydrogen atoms in eclipsed conformers which are near or in-line with one another. As had mentioned at above, the further the hydrogen atoms, the more stable the conformation due to less steric hindrance between the same
molecules.
In staggered conformations, anti-conformation has the most stable conformation because the identical groups are 180 degrees away from each other. Compare to gauche conformation, even though it is also a staggered conformer, the identical groups are only 60 degrees away from each other. In the case of fully eclipsed conformers, it is the most unstable conformation isomeric form of butane even in compare to eclipsed conformers because it has two identical carbon molecules located directly in line. This cause a strongest steric hindrance making the conformation the most unstable one. In an eclipsed conformation, even though the molecules are in line but there are not an identical group of molecules.
The overall stability order: Anti-conformation > Gauche conformation > Eclipsed conformation > Fully eclipsed conformation
According to the rotation sequence of conformational isomers of butane and the order of stability of the isomers, a free energy diagram of butane as a functional of dihedral angle can be drawn. See diagram below.
Butane, C4H10 is an alkane with four carbon atoms which are bonded to each other by single bonds. Butane can form two isomers which are n-butane and isobutane. Figure1.1(a) and 1.1(b) shows the structures of n-butane and isobutane.
Alkanes usually show conformational isomerism due the presence of C-C single bonds. It is important to understand that conformational isomer can only occur in compound with single bond but not double or triple bond. This is because only single bond allows free rotation among the atoms as it has lower energy. Compare to double bond which has higher energy that cause restriction of rotation, breaking the bond to allow rotation is mostly impossible.
The rotations of isobutane on the axis of C-C bond can be seen in many angles’ …show more content…
degrees, there are 60°, 120°, 180°, 240°, 300° and 0°, 360° and each angle of rotation give rise to different conformational isomers. For example, eclipsed, gauche, and anti-butane are all conformational isomer to each other. See Table 1.2, it shows different conformational isomers of butane with their angle’s degree.
The conformational isomers of butane can interconvert to each other around the central carbon single bond. For example, the fully-eclipsed conformation butane rotates 180° to form anti-conformation butane and further rotate 90° to form gauche conformation butane. See figure 1.3. The rotation sequence is based on 60° rotation on the C-C bond where fully eclipsed conformation is convert to gauche conformation, then convert to eclipsed conformation, followed by anti-conformation, then back to eclipsed conformation, gauche conformation, fully eclipsed conformation and so on.
The stability of the conformational isomers of butane is depends on the degrees of rotation, where the further the identical molecule groups, the more stable the conformation due to reduce of steric hindrance between the molecules.
The stability of the conformations can be explained by focusing on staggered and eclipsed conformers. Staggered conformers are with dihedral 60 degrees, and the hydrogen atoms of C-1 are far from the hydrogen atoms of C-2. Staggered conformers include anti-conformation and gauche conformation. Eclipsed conformers are which the hydrogen atoms of C-1 are lined up with the hydrogen atoms of C-2. Refer back to Table 1.2 which clearly shows the structure explain the staggered and eclipsed conformers. Staggered conformers are more stable than eclipsed conformers due to distribution of hydrogen atoms which can lead to steric hindrance. The hydrogen atoms in staggered conformers are further away from one another compare to the hydrogen atoms in eclipsed conformers which are near or in-line with one another. As had mentioned at above, the further the hydrogen atoms, the more stable the conformation due to less steric hindrance between the same
molecules.
In staggered conformations, anti-conformation has the most stable conformation because the identical groups are 180 degrees away from each other. Compare to gauche conformation, even though it is also a staggered conformer, the identical groups are only 60 degrees away from each other. In the case of fully eclipsed conformers, it is the most unstable conformation isomeric form of butane even in compare to eclipsed conformers because it has two identical carbon molecules located directly in line. This cause a strongest steric hindrance making the conformation the most unstable one. In an eclipsed conformation, even though the molecules are in line but there are not an identical group of molecules.
The overall stability order: Anti-conformation > Gauche conformation > Eclipsed conformation > Fully eclipsed conformation
According to the rotation sequence of conformational isomers of butane and the order of stability of the isomers, a free energy diagram of butane as a functional of dihedral angle can be drawn. See diagram below.